Consumption of terrestrial dissolved organic carbon by stream microorganisms

Terrestrial dissolved organic carbon (DOC) is the largest organic C pool in lotic systems, yet its role in supporting these ecosystems remains challenging to quantify. To assess the reliance of stream heterotrophic microorganisms on terrestrial DOC, this study utilized a C-13-labeled leaf leachate as a proxy for stream-water DOC of terrestrial origin. This leachate was introduced into dark, stream-water-fed bioreactors for 41 d and a suite of analytical techniques was used to evaluate community responses. Metabolic responses were quantified through measurements of DOC uptake, oxygen consumption, and bacterial production and abundance. Microbial community structure was assessed using phospholipid fatty acid (PLFA) analysis and metabolically active microbes were identified through compound specific isotope analysis. Leachate additions increased stream-water DOC concentration on average by 6% and changed its delta C-13 from -28 to +1021 parts per thousand. Leachate additions did not change microbial community structure or bacterial production and growth efficiency, but affected DOC and O-2 consumption. Prokaryotes comprised 80% of the bioreactor microbial biomass, with aerobic and facultative anaerobic bacteria dominating, and heterotrophic microeukaryotes comprising the remaining 20%. All PLFAs were enriched in C-13, indicating that the leachate moved through the bacterial community and across trophic levels. The trophic transfer of leachate C-13 to heterotrophic microeukaryotes was 32%. These findings indicate that labile terrestrial DOC plays an important role in stream metabolism, as well as supporting higher trophic levels in the microbial loop.